The present invention generally realtes to an exhaust gas purifying apparatus and, more particularly, to a dual-catalyst exhaust gas purifying apparatus for an automotive vehicle engine of a type having a plurality of engine cylinders.
The U.S. Pat. No. 3,908,371, patented on Sept. 30, 1975, discloses an exhaust gas purifying apparatus for an automotive vehicle, which utilizes a dual-catalyst exhaust treatment system including a reducing catalyst for removing an excess of oxygen and reducing nitrogen oxides in the exhaust gases and an oxidizing catalyst for oxidizing carbon monoxide and hydrocarbons in the exhaust gases. More specifically, according to the above mentioned United States patent, a plurality of exhaust ducts respectively leading from exhaust outlets of the corresponding engine cylinders are merged into a single exhaust manifold having reducing and oxidizing catalyst units disposed thereon so that the exhaust gases emitted from the engine cylinders subsequent to combustion of an air-fuel mixture within such engine cylinders can, after having been joined together, pass through the reducing catalyst unit and then through the oxidizing catalyst unit prior to such exhaust gases being discharged to the atmosphere. In order to enhance oxidization of carbon monoxide and hydrocarbons in the exhaust gases which have passed through the reducing catalyst unit, the purifying apparatus of the above mentioned United States patent further comprises a secondary air supply means including an engine operated pump for introducing a secondary air into a portion of the exhaust manifold between the reducing catalyst unit and the oxidizing catalyst unit.
In the prior art dual-catalyst purifying apparatus of a construction described above, the engine operated pump for introducing the secondary air into the manifold portion between the reducing and oxidizing catalyst units is essentially necessitated. The reason for this is obviously rooted in that all of the exhaust ducts respectively leading from the individual engine cylinders are bundled into the single exhaust manifold at a position upstream of the reducing catalyst unit in terms of the direction of flow of the exhaust gases towards the atmosphere. More specifically, as is well known to those skilled in the art, automobile exhaust gases successively emitted from the engine cylinders generally flow through an exhaust manifold towards the atmosphere in a substantially pulsating manner with the pressure inside the exhaust manifold alternately increasing and decreasing because of different timing of firing of air-fuel mixtures within the respective engine cylinders.
While so is well known to those skilled in the art, if all of the exhaust gases successively emitted from the individual engine cylinders are allowed to pass through the reducing catalyst unit such as in the prior art dual-catalyst purifying apparatus, the pulsating characteristic of flow of the exhaust gases through the exhaust manifold downstream of the reducing catalyst unit is so weakened that the prior art dual-catalyst purifying apparatus cannot make use of such pulsating characteristic in supplying the secondary air into the exhaust manifold portion downstream of the reducing catalyst unit and upstream of the oxidizing catalyst unit, and an external pumping means, such as the engine operated pump, is necessitated accordingly.
The employment of the engine operated pump substantially results in increase of the cost of manufacture of the purifying apparatus which in turn reflects upon the cost of manufacture of an automotive vehicle equipped with such purifying apparatus. In addition, since the engine operated pump utilizes a drive from a power output shaft of the engine, the power output shaft is unnecessarily loaded which substantially constitutes one of causes of reduction in power output of the engine as a whole.
The copending U.S. patent application Ser. No. 891,111, filed on Mar. 28, 1978, in which two of the inventors of the present invention are involved and which is assigned to the same assignee of the present invention, discloses a dual-catalyst purifying apparatus which substantially eliminates the disadvantages and inconveniences inherent in the above described prior art purifying apparatus. According to this copending U.S. patent application, a plurality of exhaust ports respectively leading outwards from engine cylinders are divided into first and second groups, the exhaust ports of the first group being communicated to a main exhaust manifold having reducing and oxidizing catalyst units disposed therein while the exhaust ports of the second group are communicated to a portion of the main exhaust manifold between the reducing and oxidizing catalyst units through an auxiliary exhaust manifold.
For supplying a secondary air necessary to enhance oxidization of the exhaust gases during the passage of the exhaust gases through the oxidizing catalyst unit, a portion of the auxiliary exhaust manifold is communicated to a source of secondary air through a pressure operated valve which is employed in the form of a reed valve.
The dual-catalyst purifying apparatus disclosed in the copending U.S. patent application is so designed that, since there is no substantial resistance to flow of the exhaust gases in the auxiliary exhaust manifold, the exhaust gases which are successively emitted from the exhaust ports of the second group at intervals determined by the timing of successive firing of the air-fuel mixture within the respective engine cylinders flow through the auxiliary exhaust manifold in a pulsating manner with the pressure alternately increasing and decreasing and this pulsating flow of the exhaust gases through the auxiliary exhaust manifold can be utilized to repeatedly open and close the reed valve.
The dual-catalyst purifying apparatus disclosed in the copending U.S. patent application is operable satisfactorily. However, it has been found that, during a particular operating condition of the engine, the purifying capability of the apparatus as a whole is more or less lowered as compared with that during other operating conditions of the same engine. This is particularly true when the engine starts its operation while it is cool, specifically, when the oxidizing catalyst unit is correspondingly cool and of a temperature lower than the operating temperature at which the oxidizing catalyst exhibits its performance.
In particular, when the engine starts its operation while it is cool, for example, during the cold start of the engine, an enriched air-fuel mixture is generally supplied into the engine cylinders to facilitate a quick start of operation of the engine and also to facilitate acceleration of the vehicle, the consequence of which is that exhaust gases containing a relatively great amount of noxious unburned components such as carbon monoxide and hydrocarbons are emitted from the engine cylinders. On the other hand, it is quitely natural that, when the engine is cool, the oxidizing catalyst unit is correspondingly cool.
Therefore, with the dual-catalyst purifying apparatus disclosed in the copending U.S. patent application, unless the oxidizing catalyst is heated to the operating temperature usually within the range of 250.degree. to 400.degree. C. depending upon the type thereof, the oxidizing catalyst unit would not satisfactorily serve the purpose, not completely though it may be. Moreover, the supply of the secondary air towards the oxidizing catalyst unit through the auxiliary exhaust manifold together with the exhaust gases emitted from the second group of the exhaust ports of the engine results in a prolonged period of time required for the oxidizing catalyst to be heated to the operating temperature since the secondary air introduced is substantially of an atmospheric temperature.